Downhole upgrading of oils

ABSTRACT

Oils present in oil reservoirs can be upgraded by high temperature cracking through the injection of oxygen into the reservoir and combusting the oils to generate heat. By employing injection wells and production wells, the oxygen may be placed into the reservoir increasing the flux of oxygen present as well as the temperature and cracking severity needed to produce upgraded oil.

FIELD OF THE INVENTION

[0001] The present invention relates to the injection of oxygen oroxygen-enriched air into an oil reservoir causing in-situ combustion tooccur and the oil to crack into lighter fractions.

BACKGROUND OF THE INVENTION

[0002] In conventional in-situ combustion (ISC) processes, also calledfire flooding or heavy oil air injection (HOAI), vertical wells are usedfor injection of air and typically water for the production of oil. Thedistance between the wells is often substantial and oil and watervaporized by the combustion and upgrading process condense in the coolerparts of the reservoir, travel through the heavy oil and are producedvia well techniques. Due to the highly viscous oils through which theselighter fluids must travel, it may be difficult to maintain productionand pressure may build on the injection side. This may be one reason forfailure of field applications of such technology in the past. A shortdistance process may be utilized in which a vertical or horizontalinjection well and a horizontal producer well are used so thatdisplacement of oil can be achieved along the horizontal producer well.A combustion front propagates through the reservoir above the horizontalwell allowing good communication of the upgraded oil and the productionwell. One example of such a process is the Toe to Heel Air Injection(THAI) process. A catalyst may be placed in the producer well to obtainfurther upgrading of the oil, as in the CAPRI process.

[0003] The Combustion Override Split-production Horizontal well (COSH)process also uses air injected into the reservoir to generate steam andheat in-situ. A well arrangement is used to segregate and control fluidflows and thereby reduce early oxygen breakthrough as well as sandingand gas locking of downhole pumps. The well arrangement makes use of anair injection well, gas producer well which removes excess nitrogen andother gases from near the top of the pay zone, and horizontal well torecover oil from a lower portion of the pay zone. The COSH process hasan advantage over the THAI and CAPRI processes in that problems whicharise from the handling of nitrogen and other gases are reduced.

[0004] Other examples of short distance displacement processes includeSteam Assisted Gravity Drainage (SAGD) and Vapor Extraction (VAPEX). Inthe SAGD process, steam enters through a horizontal injection well andtravels a relatively short distance to a horizontal production well. Theheating of heavy viscous oils between these wells allows the oils toflow to the production well. The VAPEX process is similar to SAGD buthydrocarbon vapor is used instead of steam. Asphaltene precipitation iscaused by the mixing of solvent and oil and provides for an in-situupgrading of the oil.

SUMMARY OF THE INVENTION

[0005] The present invention provides for a method for cracking oil inan underground oil reservoir comprising injecting oxygen into the oilreservoir and igniting the oil therein. The combustion associated withthe high influx of oxygen will generate high temperatures which willcause the oil to crack into lighter fractions to form coke orcarbonaceous solids from the heaviest compounds in the oil such asasphaltenes.

[0006] The present invention also provides for injecting oxygen into theoil reservoir such that in-situ combustion can take place for recoveringthe oil through one or more production wells.

DETAILED DESCRIPTION OF THE INVENTION

[0007] The present invention provides for an in-situ combustion processfor recovering oil from an underground oil containing reservoir. Theprocess comprises injecting oxygen into the oil reservoir. This oxygenwill react to combust the oil which causes heat generation. Theresulting high temperatures will cause the oil to crack into lighterfractions. The invention is most applicable for heavy oil or tar sands.By injecting oxygen and igniting the oil in the presence of the oxygen,high temperatures greater than 400° C. and preferably greater than 500°C. cause the oil to crack to form lighter more valuable products, aswell as coke or carbonaceous solids. In addition, higher temperaturesare desirable to facilitate the formation of CO₂. Less desirable oxygencontaining hydrocarbons may be formed at lower temperatures.

[0008] For purposes of the present invention, oxygen can mean pure 100%oxygen gas, but it can also include oxygen-enriched air which containsoxygen in an amount greater than 25%. Purification of the oxygen allowsfor a significantly higher flux of oxygen to be placed into the well dueto the reduction of associated nitrogen which would be present in air.This aids in increasing the temperature which increases the crackingseverity but also provides for reducing the need to handle nitrogen inthe gases at the production well.

[0009] In one embodiment of the present invention, a horizontal producerwell and vertical injection well is employed. The horizontal producerwell will result in a short distance displacement process whereby oiland water vaporized by the intense heat of the oil combustion front cantravel freely to the horizontal producer well. This will allow the largeflux of oxygen into the reservoir to continue unhampered by upstreamblockages. Injectivity of oxygen-containing gas is increased by thecombustion of carbonaceous materials near the injection well. In asecond embodiment, two sets of horizontal wells may be employed wherebyoxygen is injected in one set of horizontal wells and oil is producedfrom a lower perpendicular set of horizontal wells. The oxygen, wheninjected into the reservoir, may be ignited by an electronic device orother form of heat, such as steam, which will increase local temperaturein the reservoir

[0010] Water may also be injected with the oxygen once the combustionzone has been established. The steam generated in this manner is anefficient means to transfer heat to the oil. The cracked oils will be ofhigher quality in that they are relatively light, virtually free ofmetals and have a lower sulfur content than the untreated oils. Thetemperatures greater than 400° C. by which the oils are cracked willalso improve carbon dioxide production. The carbon dioxide is known toreduce viscosity and interfacial tension in the oil as well as to causeswelling of the oil in order to enhance production. The use of oxygennecessarily means that less nitrogen or no nitrogen is present in thereservoir such that a much higher flux of oxygen is provided andconsequently higher temperatures and higher concentrations of carbondioxide are present.

[0011] One means of obtaining the oxygen employed in the presentinvention is from an air separation plant which can be on-site or veryclose to the actual production wells. In preferred embodiments of thepresent invention, the well arrangement having a first vertical gasinjection well located near the top of the oil bearing portion of thereservoir and a horizontal oil production well located near the bottomof the reservoir. The oxygen would be injected into the first well,combusted and drive the cracked oils into the horizontal oil productionwell where the oil can be recovered by conventional means. In anotherpreferred embodiment, several horizontal gas injection wells runningsubstantially parallel to each other are located near the top of the oilbearing portion of the reservoir and several oil producing wells alsohorizontal and substantially parallel to each other are situatedperpendicular to the gas injection wells. This provides additionaladvantages in that the gas injection wells could be used one at a timeto upgrade the reservoir oils in the vicinity of that particular gasinjection well. The injection and production wells may be formed of anymaterial that is commonly employed in the oil production industry. Oneexample would be a perforated stainless steel tubing of dimensionssufficient to deliver oil from the reservoir.

[0012] While this invention has been described with respect toparticular embodiments thereof, it is apparent that numerous other formsand modifications of the invention will be obvious to those skilled inthe art. The appended claims of this invention generally should beconstrued to cover all such obvious forms and modifications which arewithin the true spirit and scope of the present invention.

Having thus described the invention, what I claim is:
 1. A method forcracking oil in an oil reservoir comprising injecting oxygen into saidoil reservoir and igniting said oil.
 2. The method as claimed in claim 1further comprising a production well.
 3. The method as claimed in claim1 wherein said cracked oil is recovered through said production well. 4.The method as claimed in claim 1 wherein said oil comprises heavy oiland tar sands.
 5. The method as claimed in claim 1 wherein said crackingis performed at a temperature greater than 400° C.
 6. The method asclaimed in claim 1 wherein said cracking is performed at a temperaturegreater than 500° C.
 7. The method as claimed in claim 1 wherein saidoxygen is pure oxygen or oxygen-enriched air.
 8. The method as claimedin claim 7 wherein said oxygen or oxygen-enriched air is produced by anair separation facility.
 9. The method as claimed in claim 1 whereinignition is provided by an electronic device or by steam heat.
 10. Themethod as claimed in claim 1 further comprising injecting water intosaid oil reservoir.
 11. An in-situ combustion process for recovering oilfrom an underground oil containing reservoir comprising the steps of:injecting oxygen into said reservoir through an injection well,combusting and cracking said oil; producing cracked oil, and recoveringsaid cracked oil through a production well.
 12. The process as claimedin claim 11 wherein said injection well is vertical and said productionwell is horizontal.
 13. The process as claimed in claim 11 wherein saidinjection well and said production well are horizontal.
 14. The processas claimed in claim 11 wherein said production well and said injectionwell are perpendicular to each other.
 15. The process as claimed inclaim 11 wherein there are a plurality of injection wells.
 16. Theprocess as claimed in claim 11 wherein there are a plurality ofproduction wells.
 17. The process as claimed in claim 11 wherein saidcombustion takes place at temperatures greater than 400° C.
 18. Theprocess as claimed in claim 11 wherein said combustion takes place attemperatures greater than 400° C.
 19. The process as claimed in claim 11further comprising recovering carbon dioxide from said production well.20. The process as claimed in claim 11 wherein said injection well islocated near the top of said oil bearing reservoir and said productionwell is located near the bottom of said oil bearing reservoir.
 21. Theprocess as claimed in claim 11 wherein a catalyst is present in saidproduction well.